Article history: Flash evaporation with velocity in horizontal direction, called the circulatory ﬂash evaporation, is theReceived 26 August 2012 main mechanism in many industry processes such as multi-stage ﬂash (MSF) desalination. The previousReceived in revised form 17 August 2013 studies are mainly concerned about the characteristics of static ﬂash evaporation or the ﬂow pattern ofAccepted 23 August 2013 the ﬂuid in the single stage of the MSF. Little work has been performed on the heat transfer characteristicsAvailable online 20 September 2013 of circulatory ﬂash evaporation. In the present paper a circulatory ﬂash evaporation system was built. The experiments were carried out with ﬂow rates of 400, 600, 800, 1000, 1200 L h1, initial water ﬁlm heightsKeywords: ranging from 100 to 265 mm and at pressures of 7.4, 12.3, 19.9, 31.2 kPa, respectively. Results indicatedCirculatory ﬂash evaporationHigh horizontal velocity that non-equilibrium fraction (NEF) augmented to a peak value at ﬁrst and decreased monotonouslyNEF when superheat increased. The heat transfer coefﬁcient dropped with the increase in the superheat.Volumetric heat transfer coefﬁcient The inﬂuence of ﬂow rate, pressure in the ﬂash chamber and initial water ﬁlm height on the variation of NEF and heat transfer coefﬁcient was also investigated. The NEF decreased with increasing ﬂow rate and pressure in the ﬂash chamber but increased as the initial water ﬁlm height rising. However, the vol- umetric heat transfer coefﬁcient showed an opposite law. Ó 2013 Elsevier Ltd. All rights reserved.

and non-equilibrium fraction were proposed in this article. Flash evaporation is a phenomenon of violent pool boiling. Gopalakrishna et al. [11,12] investigated the seawater ﬂash evapo-When water is channeled into low pressure environment, the li- ration with superheats ranging from 0.5 to 10 K, initial water ﬁlmquid bulk becomes superheated and its superheat will transform heights of 165,305 and 467 mm and the solution concentrationinto latent heat of ﬂash steam when the liquid temperature is high- from 0% to 3.5%. They proposed a correlation of mass evaporateder than the saturation temperature corresponding to the environ- on the parameters mentioned above. Saury et al. [13] conductedment pressure. Due to the sudden phase change, ﬂash a study on the distilled water ﬂash evaporation with superheatsevaporation phenomenon causes a sudden temperature drop of of 1–35 K, initial water ﬁlm height of 15 mm and initial water tem-the liquid. It is widely used in industrial processes such as the cool- perature from 30 to 75 °C. A correlation between the water massing of hot parts of a shuttle by water spraying under low pressure evaporated by ﬂashing and the superheat was then obtained. Theconditions [1,2], the salt disposal [3,4] and seawater desalination duration of the ﬂash evaporation phenomenon was estimated. Be-[5,6], grape cooling in wine manufacturing process [7] and the sides, studies on the circulatory ﬂash evaporation are performed,break of the core reactor cooling in nuclear power plants [8]. mainly in MSF process. Mandil and Ghafour [14] proposed a new Flash evaporation phenomenon received worldwide attention approach to the optimization of multi-stage ﬂash evaporationrecent decades. The static ﬂash evaporation was ﬁrstly studied. plants. Fath [15] simulated a 6-stage multi-stage ﬂash evaporationMiyatake et al. [9,10] carried out an experiment on pure water system. The ﬂash evaporation stage efﬁciency b was deﬁned towith superheat ranging from 3 to 5 K and equilibrium temperature evaluate the performance of each ﬂash evaporation stage. Theyfrom 40 to 80 °C. They found that the ﬂash evaporation underwent found that an increase in the superheat and the residence time in the ﬂash chamber had promoted the evaporation. Jin [16,17] per- ⇑ Corresponding author. Tel.: +86 29 82665741; fax: +86 29 82675741. formed experimental and simulation work on the singe stage of E-mail addresses: yszhang@stu.xjtu.edu.cn (Y. Zhang), wangjinshi@mail.xjtu. multi-stage ﬂash evaporation system at saturated pressureedu.cn (J. Wang), liujp@mail.xjtu.edu.cn (J. Liu), dtchong@mail.xjtu.edu.cn 0.023 MPa and 75, 97, 177 mm high water ﬁlm. Factors such as(D. Chong), zhangweijacson@stu.xjtu.edu.cn (W. Zhang), yanjj@mail.xjtu.edu.cn bubble size, distribution of the vortex number and water ﬁlm(J. Yan). height inﬂuencing ﬂash evaporation was analyzed under different 1 Tel.: +86 29 82665742; fax: +86 29 82675741. 2 superheats. They found the multiphase ﬂash ﬂow was dependent Tel.: +86 29 82665359; fax: +86 29 82675741.

on the superheat and the nucleation distribution. Our research tilled water is led into the electrical heater by the circulatingteam [18] conducted a comparative work on the heat and mass pump. It is channeled into the ﬂash chamber through the inlettransfer characteristics of static and circulatory ﬂash evaporation. valve when it is heated to the demanding temperature. Flash evap-A uniﬁed calculating model for these two ﬂash evaporation pat- oration happens as soon as the water gets into the chamber, andterns was set up as well as a new volumetric heat transfer coefﬁ- the ﬂash steam is evacuated to the condenser to be liqueﬁed andcient. Furthermore, study on the steam-carrying effect and the ﬂow rate of the condensed water is measured by mass ﬂowme-experiments on the aqueous NaCl solution at different pressure ter. Simultaneously data acquisition system starts to work.and water ﬁlm height were presented in Zhang et al.’s work The pressures are measured by absolute pressure sensors with[19,20]. range of 0–0.2 MPa and a precision of 0.25% in full scale. The tem- The previous studies mainly focused on the static ﬂash evapora- perature in this experiment is gauged by a series of T type thermo-tion and the ﬂow characteristics in the MSF process. Little work on couples. The distribution of thermocouples in the ﬂash chamber isthe heat transfer performance of circulatory ﬂash evaporation has shown in Fig. 1(b). Thermocouples above the water level are usedbeen conducted. Because of the effect of the horizontal velocity, the to measure the temperature of ﬂash vapor and the ones below theheat transfer characteristics between static ﬂash evaporation and level to the ﬂashed water in the ﬂash chamber.circulatory ﬂash evaporation is different, a circulatory ﬂash evapo- Experiments were conducted with ﬂow rates of 400, 600, 800,ration system was designed and built up. Furthermore, the effect of 1000, 1200 L h1, water ﬁlm heights ranging from 100 topressures, initial water ﬁlm heights in ﬂash chamber and ﬂow rates 265 mm and at pressures of 7.4, 12.3, 19.9, 31.2 kPa, respectively.inﬂuencing the ﬂash evaporation was studied in this paper. 2.2. Uncertainty analysis and reproducibility2. Experimental setup and methods Ranges of all parameters in this experiment are listed in Table 1.2.1. Experimental system The uncertainty of the parameters, calculated using the Moffat [21] method, is also shown in Table 2. A test rig for circulatory ﬂash evaporation, shown in Fig. 1(a), Fig. 2 illustrates the reproducibility of this experiment. It showswas designed and constructed. This apparatus contains four circu- the NEF evolution at pressure of 19.9 kPa and the initial water ﬁlmlatory loops: a basic hydrothermal loop, a ﬂash steam loop and two height of 180 mm with a ﬂow rate of 800 L h1. These two curvesauxiliary condensing loops. The basic hydrothermal loop is com- ﬁt well in the whole superheat scope. Results indicate that thisposed of a circulating pump, an electrical heater, two metal rotam- experiment has excellent reproducibility and the experiment dataeters with a range of 0–1600 L h1 and a precision of 1%, a ﬂash is authentic.chamber and a heat exchanger. The electrical heater has 20 groupsheating outside the tube. Each group has a power of 3 kW. To get 3. Results and discussionconcise temperature regulation, 3 groups of them are controlledby a voltage regulator. The ﬂash chamber is a rectangular cavity 3.1. Visualizationwith a height of 0.66 m and a cross section of b L = 0.1 0.1 m.The front of the ﬂash chamber is covered with glass plate for visu- As is shown in Fig. 3, it is visualization of ﬂash evaporation atalization just like the back. Two 25 mm-diameter adjusting valves different superheats-with pressure of 12.3 kPa, ﬂow rates ofwith two thermocouples are arranged at the inlet and outlet of the 800 L h1 and initial water ﬁlm height of 180 mm. Firstly, bubblesﬂash chamber. A liquid level meter with a precision of 0.1 mm is in the ﬂash chamber are little and have a small size. The size getsset at the right side of ﬂash chamber to measure the initial water larger and the number of bubbles is more when the superheat in-ﬁlm height. Water in the loop is driven by a circulating pump. A creases. The water ﬁlm in the ﬂash chamber is slit by the ﬂashshell-tube heat exchanger is placed near the outlet of the ﬂash steam, getting unsteady. The ﬂash vapor layer gets thicker as thechamber to cool down the water so as to protect the pump from superheat of water ﬁlm becomes larger, corresponding to the thin-cavitation. The ﬂash steam loop consists of a shell-tube heat ex- ning of the water ﬁlm layer.changer and a mass ﬂowmeter with a range of 0–110 kg h1 anda precision of 0.2%. The two auxiliary loops include centrifugalpumps, a water tank and a heat exchanger which ensures the ﬂash 3.2. NEF vs. superheat at different conditionssteam completely condensed and less leakage. The two auxiliary loops and the vacuum pump are run up to NEF at the outlet of the ﬂash chamber is a signiﬁcant indicatorcreate a stable pressure at the beginning of each experiment. Dis- to evaluate the completion degree of circulatory ﬂash evaporation.838 Y. Zhang et al. / International Journal of Heat and Mass Transfer 67 (2013) 836–842

Parameter Absolute Minimal measured Uncertainty

uncertainty value p/MPa 5 104 0.0074 6.75 102 T/°C 0.2 40.2 4.98 103 H/m 0.1 1.0 103 Fig. 2. The reproducibility of this experiment. 1 104 b/m 1 104 0.1 1.0 103 L/m 1 104 0.1 1.0 103 Q/L h1 8.0 400 0.02 mev/kg s1 – – 2.0 103 the water is 1.1 °C higher than the one at the top when the pressure hc / kW m3 K1 – – in the ﬂash chamber is 70 kPa and 6.7 °C when the pressure is 7 kPa. 8.34 102 Fig. 5((a) and (b)) indicates evolution of NEF vs. superheats at DT/K – – 4.98 103 NEF – – 8.33 102 different ﬂow rates. For all pressure, NEF ﬁrstly increased to a peak value and then decreased with the increase of superheats. More and more superheat energy was transformed into the latent heat of the ﬂash vapor when the superheats became larger. These two ﬁgures also illustrate that NEF elevated as the ﬂow rate increased.A smaller NEF corresponds to more completed ﬂash evaporation. The increase in the ﬂow rate strengthened the turbulence of theThe outlet temperature was measured in order to obtain the NEF: horizontal ﬂow in the ﬂash chamber. The water ﬁlm was slit by the ﬂash vapor so that the effect of hydrostatic head got weak. T out T e DT outNEF ¼ ¼ ð1Þ The water at lower level could evaporate. T in T e DT in Evolution of NEF vs. superheats at different pressures is pre-Fig. 4((a) and (b)) illustrates variation of NEF vs. superheats at dif- sented in Fig. 6((a) and (b)). From the ﬁgures we can conclude thatferent initial water ﬁlm heights. NEF dropped with the rising of the higher pressure results in less NEF. The higher pressure corre-the initial water ﬁlm height under the same condition after the peak sponds to the higher saturate temperature so that the initial tem-point. Increasing of the initial water ﬁlm height results in a larger perature of water into the ﬂash chamber at higher pressure ishydrostatic head which suppressing the ﬂash evaporation. Take larger than the one at lower pressure at the same superheat. Whilethe height 0.3 m for example, additional pressure (qgH) can reach the viscosity, the latent heat of vaporization and the speciﬁc heatnearly 3 kPa. As a result, the saturation temperature at bottom of capacity decrease with the increase of water temperature. Flash Y. Zhang et al. / International Journal of Heat and Mass Transfer 67 (2013) 836–842 839

evaporation is more intended to occur at higher water gradually drops down. This result can be explained as follows. Astemperature. is shown in Fig. 3, when superheat is small, corresponding to qui- All the NEF curves showed that there exists a peak value. As escent water ﬁlm, hot water quickly leaves the ﬂash chamber be-superheat increases, NEF quickly rises to a peak value and then fore the superheat is completely transformed into the latent heat840 Y. Zhang et al. / International Journal of Heat and Mass Transfer 67 (2013) 836–842

of ﬂash vapor. As superheat is large, corresponding to turbulent coefﬁcient should be introduced to evaluate the heat transferwater ﬁlm, Existing of vortexes which prolong the residence time characteristics. A volumetric heat transfer coefﬁcient [18], deﬁnedof the hot water in the ﬂash chamber so that ﬂash boiling is by Eq. (2), was adopted in our investigation.further enhanced. More superheat of hot water was transformed rmevinto the latent heat of ﬂash vapor. The bigger superheat leads hc ¼ ð2Þ 2DT in bLHto a smaller NEF. Both extreme states imply that at least a peakvalue exists. The mechanism behind will be investigated in the All the graphs below show that the heat transfer coefﬁcient dropsfuture. monotonously from a high value and no dramatic change is found when the superheat gets larger.3.3. The variation of volumetric heat transfer coefﬁcient (hc) Variation of hc vs. superheats at different water ﬁlm heights is presented in Fig. 7((a) and (b)). The heat transfer coefﬁcient in- creased when the water ﬁlm height decreased. This is due to the3.3.1. The variation of hc vs. superheats high water level which suppresses the boiling of the water at bot- As is shown in Fig. 3, the water level is unobvious and unstable tom of the ﬂash chamber as it is explained in the NEF vs. super-when the superheat increases. A reasonable heat transfer heats at different heights. Y. Zhang et al. / International Journal of Heat and Mass Transfer 67 (2013) 836–842 841

Graphs in Fig. 8((a) and (b)) illustrate variation of hc at different

pressures. There was no obvious change when the pressure altered. (a)It indicated that the pressure was insensitive to the heat transfercoefﬁcient. As is explained in the NEF evaluation vs. pressure, thepressure alteration just changed the latent heat of vaporizationand the speciﬁc heat capacity very slightly in the experimentscope. Both factors above affected the volumetric heat transfercoefﬁcient faintly. Fig. 9((a) and (b)) shows alteration of volumetric heat transfercoefﬁcient at different ﬂow rates. It could be seen that the heattransfer coefﬁcient increased when the ﬂow rates increased. Thehorizontal water ﬁlm velocity became larger when the ﬂow ratesincreased. The water turbulence became more and more violentin such a narrow 100 mm-length ﬂash chamber. This is consistentwith the law of NEF variation. (b)3.3.2. The variation of hc vs. residence time The residence time of the ﬂash evaporation in Yan et al.’s study[18] is adopted in our investigation. The expression of this time ispresented in Eq. (3)

LsL ¼ ð3Þ uFig. 10(a) shows that the heat transfer coefﬁcient decreased whenthe residence time increased. This can be explained as follows.The short residence time matched up to the large ﬂow rate. So var-iation of the heat transfer coefﬁcient with the residence time wassimilar to the inﬂuence of ﬂow rate on it. The residence time scopein the former study (area I) and this paper (area II) are presented inFig. 10(b). Experiments were carried out at pressure of 7.4 kPa, with Fig. 10. Variation of hc vs. residence time: (a) experimental results; (b) comparisonwater ﬁlm height of 150 mm and superheats of 5, 10 K. The result of theoretical and experimental results.842 Y. Zhang et al. / International Journal of Heat and Mass Transfer 67 (2013) 836–842